The objectives of the study are to demonstrate superiority, by at least 50%, of 12.5 s.c. once weekly of Ro 45-2081 over methotrexate (MTX) alone after 12 weeks of treatment in controlling the clinical manifestations of rheumatoid arthritis. The study design consists of double-blind, randomized, 6 arm, parallel-group, dose-finding, double-dummy study with a 12 week treatment period (24 twice weekly s.c. injections of Ro 45-2081/placebo and 12 weekly oral doses of MTX/placebo.